Pharmaceutical compositions in the form of particles are known, which comprise the active agent dissolved or dispersed in a polymer and which are suitable for the delayed release of some molecules of clinical interest after one single administration. Among the biodegradable polymers used for this purpose, the polylactic acid, polyglycolic acid and copolymer thereof proved to be effective in delaying the release of the drug. Also polymers of polysaccharide type have been used to modulate the release of drugs. Among those polymers xanthan, scleroglucan, ialuronic acid and chitosan proved to have good technological features and practically inexistent toxicity. Moreover some of these exhibit bioadhesive properties which render them specifically suitable to prolong the retaining time of pharmaceutical formulations in the administration sites. Both group of polymers is therefore widely used in manufacturing, for instance, microparticles comprising active agents whose release is modulated by them. Said particles systems may be prepared using techniques of emulsion whereby the active agent is dissolved or dispersed in a polymer solution and the resulting solution of dispersion is subsequently emulsified in a solvent in which the polymer is insoluble. The solvents are thereafter removed by evaporation (GB-A-2 077 692), by extraction with a second solvent (US-A-4 815 542) or by combination of the two techniques (BE-A-890 638). Moreover techniques may be found in the literature, which have been employed to modify the permeability and/of the biodegradation of the polymers which form the particle systems in such a way to exert an influence on the kinetic of the active agent release. For example there is described in the U.S. Pat. No. 4,479.911 the addition of an alkaline agent to the continuous phase used in preparing particle systems. Said agent increases the release rate of the active agent but the concentration range in which it may be used is limited by the higher degradation rate of the polymer in presence of alkaline agents. Moreover the use of strongly alkaline agents (e.g. sodium or potassium hydroxide) limits the applicability of said technique to hydrolysis stable medicaments. In the EP-A1-0 204 476 the coating of nuclei of active agent with a film consisting of a biodegradable polymer and an agent promoting the formation of pores (e.g. saccharose) is disclosed. In this case the release is influenced by the porosity generated within the film by the dissolution of said agent. This involves however the use of different excipients in preparing the active agent-comprising cope and brings about factors which may influence, in a poorly controllable way, the preparing process, the chemical-physical stability and the release kinetic of the active agent. Moreover the coating of said nucleus adds one further technological stage to the manufacturing of the end product, which is a drawback from the view point of the production. In the EP-A.sub.2 -0092 918 and JP 2078 629 there is described the preparation of biodegradable block-copolymers constituted by a biodegradable hydrophobic part and an amphiphilic part capable of absorbing water and/or biological fluids to form a hydrogel able to control the release of the active agent. Such a technology involves the development of a new copolymer whose chemical nature is different from the "sum" of the chemical features of the starting products. Accordingly the biocompatibility properties of the end product are modified as well; it should moreover be considered that the usage of said product depends on the compliance with the protocols for the clearance of new products. In the EP-A.sub.2 -O 052 510 and U.S. Pat. No. 4,675,189 there is described a process for preparing particles which involves the presence of one or more agents modifying the polymer hydrolysis in order to modulate the release of the active agent. In this case too, the presence of said agents limits the possibility of application of said technique to hydrolysis stable medicaments. Finally the preparation of a polymer mixture by co-solubilizing homo- and copolymers belonging to the polyester class is disclosed in the EP-A.sub.1 - 0 281 482; such a mixture has a degradation rate higher than the single polymers. The application of said technique is however limited to the high molecular weight medicaments only, since the permeability of the polymer is not modified, thus the diffusion of low molecular weight medicaments is not modulated. Furthermore, because of the nature of the employed polymers (high molecular weight), such a technique limits itself with regard to the relative percentage of the employed polymers, consequently affecting the flexibility of the end system. The methods according to the aforementioned literature essentially try to modify the permeability or the degradation rate of the polymer in order to modulate the rate of release of the medicament. Said methods bring about different drawbacks, such as:
addition of agents which may modify the molecular weight of the polymer, giving rise to the degradation of the same in noncontrollable way, already in the preparation phase of the microparticles; PA1 addition of several stages in the preparation process of the microparticles; PA1 synthesis of novel copolymers whose biocompatibility properties may dramatically change with respect to the starting homopolymers, since the chemical nature of the polymers is modified; PA1 addition of chemical agents which may affect the stability of the active agent within the microparticles of in any case which may limit the applicability of the technology. PA1 the co-solubilisation of the polymeric constituents with the agent modifying the interface properties in presence of solvents, if any; PA1 the solution or suspension of the active substance in the mixture of polymeric compounds; PA1 the formation of particles consisting of the polymers, the agent modifying the interface properties and the active substance by techniques of emulsion of extrusion of spray-drying of spray-congealing. The thus obtained particles exhibit improved biocompatibility properties and allow a delayed release of the active substance.